Probing nanotube-nanopore interactions
Original entry by Andrew Capulli, AP225 Fall 2011
King and J.A. Golovchenko, "Probing nanotube-nanopore interactions", Physical Review Letters 95, 216103 (2005).
The investigation into the properties of nanopore channels in biological membranes is not well characterized in their purest form; this is to say that high electrophoretically driven molecular speeds are required for the translocation of molecules through these biological nanopores (such as proteins, DNA, etc). With the semi-recent advancement in nanotechnology of scanning tunneling microscopy (STM) and atomic force microscopy (ATM) for example, as noted by the authors, there is a new interest in the physics of molecules at this size scale and the interactions thereof; in particular, at this size scale, the investigation into single molecule systems and characterization of the molecule is of interest. However, as noted, much of the study into these nanopore systems requires electrical charge of molecule in question to 'drive' its translocation through the nanopore. This addition of a charge can be thought of as slightly modifying the molecule thus resulting in study of an inexact system (not necessarily the molecule in its purest, uncharged form). This drives the need for a system of trans-nanopore delivery of molecules in such studies and is what Golovchenko et al present in this methods paper.
Using the technique of ion sculpting [J. Li et al., Nature (London) 412, 166 (2001)] nanopores of the desired diameter were constructed. The nanopore was fixed into the system (as shown below in FIG 1, separating two reservoirs a uniform ionic solution (1 M KCl, 10 mM TRIS-HCl,1 mM EDTA) which allowed current to flow between the silver (Ag/AgCl) electrodes of the system present in each reservoir (seen in black in the figure FIG 1 below). To summarize, the nanopore provides the only connection between the charged solution in the top and bottom reservoirs. An AFM cantilever with a carbon nanotube affixed to the tip of the cantilever is positioned above the nanopore and can be moved translationally (across the x-y plane the nanopore is in) and vertically (the 'z' direction through the nanopore) via the the AFM cantilever control system. Golovchenko et al refer to the movement in the z direction through the pore as being controlled by Zpiezo; this will appear in figures below and is just the vertical controlling movement of the AFM unit. Figure 1 below shows the construct:
Carbon nanotubes were fixed to the silicon AFM tips via iron catalyzation and can be seen in FIG 2 (b) below. Also in the figure are transmission electron microscopy (TEM) images of the nanopore (a) viewed along the previously discussed 'z' dirction (above the pore) as well as 'in fluid', meaning in reservoir, images of the nanopore (c) and the ionic current (d):